The present invention relates to a two-stage helical screw compressor that includes liquid injection in both stages, wherein the compressor includes a male and a female rotor in each stage and a connecting duct which is disposed between the outlet in the first compressor stage and the inlet in the second compressor stage and through which gas that has been compressed in the first stage is delivered to said second stage. The invention also relates to a method of cooling a two-stage compressor in which liquid is injected into both stages and gas that has been compressed in the first stage is delivered to the second stage through a duct connecting between the outlet in a first compressor stage and the inlet in a second compressor stage.
It is known to use a liquid, for instance a lubricant such as oil, to cool the compressor and therewith the compressed gas, to lubricate and seal the helical rotors against the rotor housing. For this purpose this liquid in finely divided form is injected into the working chambers of the compressor.
When using multi-stage compressors, the liquid-containing gas is passed from one stage to the next stage via a connecting duct. The hot oil-containing gas obtained from the first compression stage is delivered to the second compression stage, wherewith the temperature of the oil present in the gas has also been raised. The temperature of the oil is raised still further in this second stage. Consequently, in order to cool the compressor it is necessary to inject cold oil into this other stage, in the same way as that carried out in the first stage.
When there is no need to run the compressor at full capacity, the compressor stages are relieved of load, normally with the aid of a slide valve or a radially disposed lifting valve that generates a short circuit between the compressor working chambers, i.e. interconnects two mutually adjacent or juxtaposed working chambers. This destabilises the secondary drive rotor, most often the female rotor. This destabilisation results in xe2x80x9cchatteringxe2x80x9d of the rotors and also in rotor wear and loud noise.
The object of the present invention is to provide for improved cooling of two-stage compressors and therewith also improved compression in such compressors, by ensuring that most of the lubricant delivered to the second compressor stage has a low temperature.
This object is achieved with a helical screw two-stage compressor in which a pump functions to transport liquid precipitate present in the connecting duct through a heat exchanger and delivers the liquid cooled in the heat exchanger to the two compressor stages. The object is also achieved by a method which includes separating in the connecting duct the liquid injected in the first compressor stage and cooling the separated liquid in a heat exchanger and thereafter injecting the cooled liquid into both compressor stages.
Advantageous embodiments of the inventive compressor and inventive method will be apparent from the claims.
The rotor that is driven directly by drive means in a helical screw compressor is normally the male rotor. The female rotor is caused to rotate by the driven-male rotor. When relieving the compressor of load, so that the compressor runs in an idling mode or delivers only a partial load, there is a tendency for the secondarily driven rotor to destabilise and begin to chatter or rattle. This results in undesirable noise and also in wear on both rotors. By allowing the secondarily driven rotor to drive the pump that circulates the lubricating and sealing liquid, the secondarily driven rotor will be subjected to load and therewith stabilise and no longer rattle. The female rotor in the first stage will preferably drive the circulation pump.